Patent GB-897100-A (GB)
Title: Improvements in data exchange systems
Inventor: FREEDMAN HAROLD PHILIP
IPC: G01S13/937
Filed: 1958-04-11
Abstract: 897,100. Radio signalling; pulse modulation systems; pulse radar. AVEL CORPORATION GENEVA. April 2, 1959 [April 11, 1958], No. 11602/58. Classes 40(5) and 40(7). In a system in which the value of a data item, e.g. the height of aircraft, is exchanged between an interrogating station and a responding station, the data item is represented by means of a two-part code, interrogation is performed by a signal representing one of the code parts of an assigned value of the data item, the responding station includes means for determining whether or not identity exists between the interrogation code-part transmitted and the corresponding code part representing the actual value of the data item at the responding station, and if such identity exists, a response signal representing the other code-part of the actual value of the data item is transmitted to the interrogating station. In one embodiment described an altitude range is divided into coarse divisions C, D, E &c. Fig. 2, each coarse division being sub-divided into cyclically repeating fine divisions 1-10. To determine the presence of aircraft in neighbouring height zones, an interrogating aircraft at height p transmits a code signal representing its fine height value 2. An aircraft at height r whose fine height value is also 2 accepts this code signal and responds by transmitting its coarse code-part D. The interrogating aircraft accepts this signal and an indication is given of another aircraft in the same height zone. The coding arrangement employed in this embodiment represents a code division by the interval between two pulses, the interval being either continuously variable or discretely variable as shown in Figs. 3, 4. At the interrogating aircraft, Fig. 3, switches 14-16 are driven by a shaft 5 coupled to an altimeter so that the effective length of delay line 13 is dependent on the fine height value of the aircraft. Switch 16 operates relay 31 so that only one of switches 14 or 15 is selected at a time. A pulse from generator 9 keys transmitter 11 to transmit a first R.F. pulse followed by a delayed pulse. The first pulse received by the responding aircraft, Fig. 4, after traversing delay line 40 reaches coincidence gate 39 at the same time as the delayed pulse supplied directly only if the fine height values of the aircraft are the same. The output of gate 39 keys transmitter 54, operating on a different frequency to transmitter 11, to transmit a first pulse followed by a delayed pulse, the interval between the pulses being determined by the coarse height value setting of delay line 56. If the coarse height value of the interrogating aircraft is the same the response signal is accepted by means of delay line 79 and gate 78, the output of gate 78 being applied to a detector and indicator 93. A response may be obtained from aircraft which are in adjacent height zones by making contacts 43, 44 of switches 41, 42 in the responding aircraft, of sufficient width to select two adjacent tappings of delay line 40, thereby providing a spread of acceptable fine height values. Specification 874,261 is referred to.